This invention relates to a method of producing thresholded height-related images of objects and to an integrated lighting/imager system for computer vision applications.
In image processing operations for robotics, inspection, and other machine and computer vision tasks, it is often wished to develop a high contrast "shadowgram" or silhouette of a machine part lying on a plane surface such as a conveyor belt. Developing a true silhouette by using backlighting (light tables, fluorescent conveyors, etc.) is generally impractical in industrial settings and normal front lighting and simple video thresholding do not give good quality images. Too often, good contrast depends on proper background color and reflectivity, both of which are seldom optimum for all type parts in all situations. Shadows and specular reflections further confuse the scene after thresholding.
An approach that has been taken is to use structured light to form a scene description. When a sheet of light impinges on a three-dimensional surface, the light bands are distorted from their initial appearance due to their interception by the irregular surface. The prior art system in FIGS. 1a and 1b views parts on a moving conveyor, and has two fixed sheets of light intersecting in a fixed line and a line scan array to build a silhouette strip-by-strip as a precisely moving conveyor transports the part step by step through the light bar.
Line sources LS1 and LS2 emit sheets of light which are caused to intersect in a line L on the conveyor surface, and a one-dimensional array of photosensitive elements LC is focused along that line on the conveyor. As the conveyor causes the object to translate through the light bar/array focus line, the non-zero height of the object causes the image of the light line to break up and those portions impinging on the object are displaced left or right by an amount proportional to the height of the surface. The portion not hitting the object remains along the conveyor intersection line. Thus, line segments L1 and L2 are parts of original line L while L3 and L4 are shitted images of the left and right sheets of incident light. The line array is scanned for each object position and the process repeats. After a large number of discrete steps the object silhouette is built up.
The major drawbacks with this approach are the need for precise conveyor positioning; the need for no slippage of the object on the belt or no movement of subassemblies during "exposure"; long exposure times, as much as 5-10 seconds with normal conveyor speeds and object sizes; and the need to organize processing around a slowly produced, sequentially scanned image.